Numerous commercial processes require the removal of suspended solids from fluid streams prior to their recycle or emission into the environment. For example, coating operations, such as electrostatic and powder coating processes, produce effluent streams containing suspended solids.
Manufacturers commonly apply either electrostatic or powder coatings to the surfaces of metal parts, such as parts for appliances and the like. The parts must be cleaned prior to coating because manufacturers generally apply oils and lubricants to the surfaces of the parts to aid in mechanical processing. Although useful during the earlier stages of manufacture, these oils and lubricants are detrimental to any coatings which can be subsequently applied. Therefore, electrostatic and powder coating processes utilize a series of wash and rinse baths prior to coating. Further, a phosphate primer can be applied immediately prior to coating, such as a primer applied from a bath containing zinc or iron phosphate. The baths, particularly the wash and rinse baths, become contaminated over time, leading to off-quality production.
To reduce the level of contamination, a portion of each of the baths is continuously removed as an effluent stream and replaced with fresh solution. The effluent stream exiting the baths must be treated to remove the contaminants, particularly heavy metal contaminants, prior to its emission to the environment. The removal of heavy metals from industrial effluent streams is particularly problematic in light of strict governmental regulations specifying stringent levels of allowable residual heavy metals in effluent water streams emitted to the environment. Particularly stringent regulations have been established for heavy metals deemed harmful to humans, such as mercury, cadmium, zinc, copper, lead, nickel and silver. For example, regulatory agencies have introduced stringent lead standards, resulting in a demand for new treatments that are able to remove lead to extremely low levels (<0.1 ppm in many cases).
Conventional wastewater treatment processes generally remove contaminants via a series of discrete chemical processes, each performed in a separate tank. For example, a conventional waste-water treatment process can consist of a series of tanks in which oils and lubricants are floated to the surface of a first tank and a number of flocculants are then added in a further series of settling tanks at different pHs to gradually precipitate the heavy metals out of the effluent. Following precipitation, the heavy metals are pumped out of the bottom of the tanks as sludge.
Many solids are slow to settle and thicken in their liquid media by gravity alone, further exacerbating the problems involved in their removal from effluent streams. Settling times of up to 3 hours are common, and it is thus frequently necessary to build relatively large settling tanks to accomplish good separation. Therefore, due to the number and size of the various tanks involved, conventional wastewater treatment processes require a significant amount of floor space. In addition, conventional wastewater treatment processes cannot process effluent streams at high flow rates. Treatment rates of about 9 gallons per minute are common, for example. In contrast, the coating processes typically supply the effluent at a much greater volumetric rate. Therefore, due to this unfortunate combination of low treatment rates and high floor space, coating processes have heretofore been forced to either shut down temporarily to allow their wastewater treatment process to catch up, coat only on a single shift but treat wastewater around the clock, or ship a portion of the effluent stream off-site for processing.